Inhibition of parathyroid hormone-induced fetal rat bone resorption in vitro by nerve growth factor

The effects of 7S nerve growth factor (NGF) and its isolated α, β, and γ subunits on bone resorption were assessed in a tissue culture system in which the degree of resorption was determined by measuring the release of⁴⁵Ca from prelabeled fetal rat radii and ulnae. It was found that 7S-NGF, through the activity of its γ, subunit, inhibits parathyroid hormone (PTH)-stimulated but not-unstimulated bone resorption. The following observations suggest that γ-NGF, a trypsin-like molecule, blocks PTH-induced bone resorption by enzymatic degradation of PTH: (a) γ-NGF does not inhibit bone resorption stimulated by the steroid, 1,25-dihydroxycholecalciferol; (b) trypsin is as effective as γ-NGF in inhibiting PTH-stimulated bone resorption; (c) the PTH-inhibitory action of both γ-NGF and trypsin are eliminated by inactivating these enzymes with diisopropyl fluorophosphate; and (d) addition of γ-NGF to the cultures 2 days after the inclusion of PTH does not result in inhibition of bone resorption. Similarly, when the subunit is added to the culture medium before the hormone, there is no inhibition of resorption. The latter observation suggests that the target of γ-NGF is the PTH molecule rather than its membrane receptors. Crystalline bovine insulin inhibits the γ-NGF suppression of PTH-induced bone resorption. This effect, however, is not mimicked by the addition of zinc, which is present in commerical insulin preparations, to the culture medium. Consequently, insulin must inhibit NGF by some mechanism other than the influence of zinc.